The present invention relates to a method for calendering a paper web, wherein the paper web is passed through a calendering nip formed by two rolls, at least one of the rolls being soft-faced by means of a coating of a polymer or equivalent material or by means of a band passed over the roll and running through the nip. The profile of the calendering nip is thereafter regulated in order to compensate for any flaws present in the web which enters the calender.
The invention also relates to a calender that comprises a calendering nip formed between two calender rolls, wherein at least one of the calender rolls is provided with a soft surface layer of a polymer or equivalent material in the form of a coating or a band passed over the roll running through the nip. The calender is provided with regulation devices for regulating the profile of the calendering nip in order to compensate for any flaws present in the web that enters the calendering nip.
It is known in the art of calendering that variations occur in the paper web entering a calendar that originate in the wet end and the dryer section of a paper machine. It therefore must be possible to control the operation of the calender, i.e., the profile of the calendering nip must be adjustable in order to compensate for these defects in the paper web.
Conventionally, the profile adjustment that takes place in the calender has been carried out by varying the diameter of a nip roll locally. The diameter has been conventionally varied, for instance, by locally heating the mantle of a metal roll or by cooling it correspondingly. Such has been accomplished by, among other things, the use of hot-air-blowing, infrared heating and different electric heating members.
However, disadvantages occur while using this method. For example, it is overly time consuming to cool a heated section of the roll. In addition, due to the fact that the mantle of the roll is most often formed of metal, it is difficult to locally heat a section of the roll because the heat is often conducted in the mantle. Moreover, when heating different sections of the roll, it is necessary to heat each section at a different temperature (the difference being relatively high). This, in turn, affects the gloss of the paper, i.e., the paper web may have more glossy or less glossy streaks after calendering.
A roll adjustable in zones represents a newer technology in the regulation of the calender profile. However, this construction does not allow the use of the narrow adjustment zones discussed above.
A method as described above is shown, for example, in U.S. Pat. No. 4,658,716, which discloses a calendering roll having a plurality of infrared heaters arranged thereon in the axial direction of the roll. The infrared heaters adjust the diameter of the calendering roll in order to compensate for the variations that occur in the transverse direction of the paper web entering the calendering nip, i.e., the heaters adjust the profile of the calendering nip by heating the metal roll externally.